feat: Integer update

[novusnota]

Adds a new page to the Book — "Integers", which describes integer literals, serialization as tl-b types and more.

Closes #60
Closes #61

The page makes couple of references to the new page added there: #95

[anton-trunov]

Awesome stuff! Left a few comments

[novusnota]

Added:

• Info on limited use of underscores (no duplicate, no trailing)
• Info on undesirable use of a leading zero in decimals
• An example of the compute phase error when working with integers and their serialization

Changed:

• defining-types is now composite-types, as that page also specifies how to use Structs and Messages
• Made a clean-up of the composite-types page
• types got a major clean-up and inter-linking with other parts of the Book. Additionally, added a sub-section for booleans right on it to not introduce a separate page for them. Motivation: in many languages booleans are used interchangeably with ints, so I felt that their description is closely related to the scope of this PR.